SA Node and AV Node

Electrical pulses in the heart are controlled by special groups of
cells called nodes. The SA (sinoatrial) node generates an electrical signal
that causes the upper heart chambers (atria) to contract. The signal then
passes through the AV (atrioventricular) node to the lower heart chambers
(ventricles), causing them to contract, or pump.

The SA node is considered the pacemaker of the heart. Its
electrical signals normally cause the atria to contract at a rate of 60 to 100
times a minute.